I should have tested that change first, but before my test ride I also lubed my seatpost frame clamp bolt and retightened that. (It had been a bit hard to turn so I wasn't sure that it was tight enough but I didn't want to force it.) Anyway... one or both of those changes stopped the creak.
Today's fettling was just cleaning a selection of old sprockets from a bag in my junk box. I am going to turn my singlespeed bike into a 4-speed bike and need the cogs for it. I found a surprising number which might be okay for the bike. Now that they are clean I will have to put my specs on and check for wear. I can see some, but generally they look pretty good. I reckon that they should last me years.
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I already have a 29-toother for my bailout climbing gear. I will be using a 15 for my standard gear and have 5 there to choose from. My highest gear will be one of those 12-toothers, which are needed for the cassette lock ring to tighten against so I might as well bring it into use. (There is one on the bike already but I can't use it.) I don't really need a 4th gear but to get the best possible chainline for my 15 I need to have another cog betweeen the 12 and the 15. I have a choice of a 13 (2 available) or a 14 (3 available) and will probably opt for the 14 to use when the 15 feels just a little bit too easy. If the 15 feels way too easy then I could use the 12.